> From: Jason Gunthorpe <jgg@xxxxxxxxxx> > Sent: Friday, August 2, 2024 7:22 PM > > On Fri, Aug 02, 2024 at 08:26:48AM +0000, Tian, Kevin wrote: > > > From: Jason Gunthorpe <jgg@xxxxxxxxxx> > > > Sent: Thursday, June 20, 2024 10:34 PM > > > > > > On Thu, Jun 20, 2024 at 04:14:23PM +0200, David Hildenbrand wrote: > > > > > > > 1) How would the device be able to grab/access "private memory", if > not > > > > via the user page tables? > > > > > > The approaches I'm aware of require the secure world to own the > IOMMU > > > and generate the IOMMU page tables. So we will not use a GUP approach > > > with VFIO today as the kernel will not have any reason to generate a > > > page table in the first place. Instead we will say "this PCI device > > > translates through the secure world" and walk away. > > > > > > The page table population would have to be done through the KVM path. > > > > Sorry for noting this discussion late. Dave pointed it to me in a related > > thread [1]. > > > > I had an impression that above approach fits some trusted IO arch (e.g. > > TDX Connect which has a special secure I/O page table format and > > requires sharing it between IOMMU/KVM) but not all. > > > > e.g. SEV-TIO spec [2] (page 8) describes to have the IOMMU walk the > > existing I/O page tables to get HPA and then verify it through a new > > permission table (RMP) for access control. > > It is not possible, you cannot have the unsecure world control the > IOMMU translation and expect a secure guest. > > The unsecure world can attack the guest by scrambling the mappings of > its private pages. A RMP does not protect against this. > > This is why the secure world controls the CPU's GPA translation > exclusively, same reasoning for iommu. > According to [3], " With SNP, when pages are marked as guest-owned in the RMP table, they are assigned to a specific guest/ASID, as well as a specific GFN with in the guest. Any attempts to map it in the RMP table to a different guest/ASID, or a different GFN within a guest/ASID, will result in an RMP nested page fault. " With that measure in place my impression is that even the CPU's GPA translation can be controlled by the unsecure world in SEV-SNP. [3] https://lore.kernel.org/all/20240501085210.2213060-1-michael.roth@xxxxxxx/
